A power circuit, in particular DC (Direct Current)-power circuits that work with high voltages and high currents, for example, in photovoltaic systems or systems providing off-grid power supply, are prone to the development of electric arcs. Electric arcs can, for example, occur when a power line with a high current load is interrupted during maintenance or in case contacts at interconnectors are degraded. Other possible causes for arc faults, i.e. the occurrence of an electric arc in the power circuit, are corroded solder joints or broken insulators of the power lines. Arc faults are the most common fire causes in photovoltaic systems. This also reflects in the requirements for arc fault protection for photovoltaic systems as, for example, regulated by the national Electric Code (NEC) 690.11 of the United States of America implemented in 2011.
A reliable arc fault detection method and system is therefore of major importance. On the one hand, for security reasons the existence of an arc fault has to be detected with a reliability as high as possible. On the other hand, the probability of an erroneous indication of a supposed arc fault has to be as low as possible, in particular if an erroneous detection of an arc fault might lead to a shutdown of a photovoltaic system without the option to automatically restart it, as for example specified in the before mentioned NEC 690.11 code.
Electric arcs usually emit a broadband AC (Alternating Current)-signal in an RF (Radio Frequency)-frequency range. Detection systems for arc faults that are based on detecting a according radio frequency signal in the power circuit are well established and known from document WO 95/25374, for example. A problem associated with the detection of arc faults via their current signature in the RF-frequency range is that disturbing RF-signals might be present in a power circuit that are misinterpreted and are wrongly assigned to arc faults, which finally leads to a nuisance alarm situation. Possible sources of disturbing signals are for example RF-radio transmitter, electric trains or trams passing by, electric or electronic devices with an insufficient electro-magnetic shielding or interference suppression, or arcs in adjacent power systems. In the following, all possible RF-sources other than an arc fault in the respective power system are hereinafter referred to as a “disturber”.
In order to increase the detection reliability document U.S. Pat. No. 7,633,727 B2 discloses an arc fault detection system comprising two narrowband band pass filters operating at different frequencies. An arc fault is determined to be existent only if a radio frequency signal is observed on both frequency bands of the two band pass filters. However, in case a disturbing signal shows a frequency spectrum that is similarly broad as the typical frequency spectrum of an electric arc, this disturbing signal cannot be distinguished from a signal arising from an arc fault.
It is thus desirable to create a robust and reliable method and system for detecting arc faults in a power circuit. It is furthermore desirable to describe a photovoltaic system with a corresponding detection system.